1. Edit eclipse-home/eclipse.ini to be something like the following and restart Eclipse.
-vmargs -Xms64m -Xmx256m
2. You can just run eclipse command with additional options at the very end. Anything after -vmargs will be treated as JVM options and passed directly to the JVM. JVM options specified in the command line this way will always override those in eclipse.ini. For example,
eclipse -vmargs -Xms64m -Xmx256m